The present invention relates to circuit protection. More particularly, the present invention relates to an improved voltage variable material (“VVM”).
Electrical overstress transients (“EOS transients”) produce high electric fields and high peak powers that can render circuits or the highly sensitive electrical components in the circuits, temporarily or permanently non-functional. EOS transients can include transient voltages or current conditions capable of interrupting circuit operation or destroying the circuit outright. EOS transients may arise, for example, from an electromagnetic pulse, an electrostatic discharge, lightning, a build-up of static electricity or be induced by the operation of other electronic or electrical components. An EOS transient can rise to its maximum amplitude in subnanosecond to microsecond times and have repeating amplitude peaks.
The peak amplitude of the electrostatic discharge transient wave (“ESD event”) may exceed 25,000 volts with currents of more than 100 amperes. There exist several standards which define the waveform of the EOS transient. These include IEC 61000-4-2, ANSI guidelines on ESD (ANSI C63.16), DO-160, and FAA-20-136. There also exist military standards, such as MIL STD 883 part 3015.
Voltage variable materials (“VVM's”) exist for the protection against EOS transients, which are designed to rapidly respond (i.e., ideally before the transient wave reaches its peak) to reduce the transmitted voltage to a much lower value and clamp the voltage at the lower value for the duration of the EOS transient. VVM's are characterized by high electrical resistance values at low or normal operating voltages. In response to an EOS transient, the materials switch essentially instantaneously to a low electrical resistance state. When the ESD event has been mitigated these materials return to their high resistance state. These materials are capable of repeated switching between the high and low resistance states, allowing circuit protection against multiple ESD events.
VVM's also recover essentially instantaneously to their original high resistance value upon termination of the ESD event. For purposes of this application, the high resistance state will be referred to as the “off-state” and the low resistance state will be referred to as the “on-state.” EOS materials can withstand thousands of ESD events and recover to desired off-status after providing protection from each of the individual ESD events.
Circuit components utilizing EOS materials can shunt a portion of the excessive voltage or current due to the EOS transient to ground, protecting the electrical circuit and its components. The major portion of the threat transient, however, is reflected back towards the source of the threat. The reflected wave is either attenuated by the source, radiated away, or re-directed back to the surge protection device which responds with each return pulse until the threat energy is reduced to safe levels.
One VVM for providing protection against electrical overstress is disclosed in U.S. Pat. No. 6,251,513 B1, entitled, Polymer Composites for Overvoltage Protection, assigned to the assignee of this invention, the teachings of which are incorporated herein by reference. Other voltage variable materials, the teachings of which are incorporated herein by reference, include the following.
U.S. Pat. No. 2,273,704, issued to Grisdale, discloses granular composites which exhibit non-linear current/voltage relationships. These mixtures are comprised of conductive and semiconductive granules that are coated with a thin insulative layer and are compressed and bonded together to provide a coherent body.
U.S. Pat. No. 2,796,505, issued to Bocciarelli, discloses a non-linear voltage regulating element. The element is comprised of conductor particles having insulative oxide surface coatings that are bound in a matrix. The particles are irregular in shape and make point contact with one another.
U.S. Pat. No. 4,726,991 issued to Hyatt et al., discloses an EOS protection material comprised of a mixture of conductive and semiconductive particles, all of whose surfaces are coated with an insulative oxide film. These particles are bound together in an insulative binder. The coated particles are preferably in point contact with each other and conduct preferentially in a quantum mechanical tunneling mode.
U.S. Pat. No. 5,476,714, issued to Hyatt, discloses EOS composite materials comprised of mixtures of conductive and semiconductive particles sized to be in a 10 to 100 micron range. The materials also include a proportion of 100 angstrom sized insulative particles. All of these materials are bonded together in an insulative binder. This invention includes a grading of particle sizes such that the composition causes the particles to take a preferential relationship to each other.
U.S. Pat. No. 5,260,848, issued to Childers, discloses foldback switching materials which provide protection from transient overvoltages. These materials are comprised of mixtures of conductive particles in the 10 to 200 micron range. Semiconductor and insulative particles are also employed in these compositions. The spacing between conductive particles is at least 1000 angstroms.
Additional EOS polymer composite materials are also disclosed in U.S. Pat. Nos. 4,331,948, 4,726,991, 4,977,357, 4,992,333, 5,142,263, 5,189,387, 5,294,374, 5,476,714, 5,669,381 and 5,781,395, the teachings of which are specifically incorporated herein by reference.
These known voltage variable materials are particularly suited for protecting data transmission signals. The growth in the data communications and telecommunications industries has provided a myriad of opportunities for these materials. One limitation keeping the known VVM's from reaching their full potential is that they are not structurally free standing. That is, the materials are typically applied to a substrate, which is then electrically connected to a use point, usually a printed circuit board (“PCB”). VVM applications are thus limited to the smallest possible substrate that will hold or support a quantity of VVM, which may be too large, the wrong shape or the wrong configuration for certain applications, e.g., for certain fine pitch integrated circuits. Other applications, such as data cabling, do not provide a convenient place to mount a substrate.
A need therefore exists to provide a more flexible and space saving way to use voltage variable materials. A need exists for the voltage variable material to be free standing so as to eliminate the need for a carrier substrate or daughter board. A need also exists for a way to electrically couple VVM's in applications that do not provide a convenient place to mount the carrier substrate.